The word racist is ubiquitous, but the word race is forbidden (expect for check boxes on government forms). There was Saturday Night Live Episode years ago of a psychologist talking with Eddie Murphy about word associations, eventually using nothing but racially-loaded terms like "spearchucker." Murphy started out affable but grew more irritated, responding with things like "cracker," and "honky." Eventually, when quite angry he said "HONKY honky!!" It was a good illustration that there aren't equivalent words to insult white people with. Except, as I read a few years ago from a black humorist, the word "racist" is as insulting to white people as any previous slur against blacks, as measured by the amount of anger and energy put into defense against it.
Because race is so emotionally-loaded, I have read suggestions that it be avoided altogether in favor of more neutral terms such as ancestry group. I don't think it will do the least bit of good, as you will just be accused of Scientific Racism with quick reference to Nazis, rather than American slavery, but I have
no little objection. We are already in a maze of contradictory statements, language-change won't make that much worse or better.
Can we blame Lewontin for his 1960s declaration that races don't exist and everyone who says they do is racist? He lent the weight of his white lab coat to that political declaration. He's up there for the nomination.
About twenty years ago the idea came in that race is a social construct, an idea that did not exist until about 500 years ago and the Age of Exploration. That seems calculated to conjure images of slaveships rather than discuss real history. People in Roman times and earlier certainly noticed the different colors of people. Until a hundred years ago, race was an even more restrictive term, referring to the Irish race, or the Slavic. So in some sense, yes, race is somewhat socially defined. The categories in the US Census have changed every time since 1970, for example.
Yet these are always definitions at the margins. Where we draw the lines are indeed a matter of opinion. Where do the Appalachian Mountains begin? When did Rome fall? What is a socialist? Is that color teal or green? But that does not mean there is no reality deeper in. I know plenty of people of mixed White/Oriental (or Caucasian*/Asian if you prefer) ancestry, but this is new. America has lots of mixes, but even here, there is remarkable continuity. If you look at the junk DNA, without reference to any skin color or facial features, you can tell what continent someone's ancestors lived on before 1500 for thousands of years back. What you say about Afghanis vs Mongols, or North Indians vs South, or border Mexicans vs Oaxacans is interesting - indeed, fascinating and the source of much additional information. But failing to acknowledge that Scots are genetically distinct from Koreans is just silly. And most of humanity still falls into the hard categories, not the fringe ones.
*Hah! Like I have significant Azerbaijani ancestry.